The invention relates to separators for electrochemical cells, particularly separators for alkaline cells, which cover an electrode surface, wherein the separator is formed by treating a film applied to the electrode surface. The invention also relates to a protector disk contacting the top surface of the cathode to protect the cathode from shorting.
Primary (non-rechargeable) alkaline cells typically contain an anode comprising active material of zinc, alkaline electrolyte, a cathode comprising active material of manganese dioxide, and an ion permeable separator sheet between the anode and cathode. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cell contents are typically housed in a cylindrical steel casing (housing). The anode material comprises zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte solution. The gelling agent holds the zinc particles in place and in contact with each other. The cathode material comprises manganese dioxide and small amount of electrolyte and may also include small amounts of carbon or graphite to increase conductivity. The cathode material is conventionally a solid material compressed against the inside surface of the cell casing (housing) forming a hard compacted annular mass.
The separator is conventionally premanufactured outside of the cell. The separator is typically cut into two or more strips and inserted in the cell casing so that the strip edges overlap. The separator material covers the inside surface of the cathode and lies adjacent the anode. Additional electrolyte may optionally be added after the separator is inserted. In such case the separator absorbs the additional electrolyte and a portion of the electrolyte is reabsorbed into the cathode. Anode material may then be inserted into the core of the casing. The separator keeps the anode and cathode from physical contact.
The separator may be of woven or nonwoven ion permeable material. The separator is ion permeable, but yet prevents passage of active anode or cathode material therethrough. Conventional ion permeable separators for alkaline cells may be formed of a single sheet of fibrous woven or nonwoven material. Such single sheet is prefabricated outside of the cell and inserted into the cell, typically in overlapping strips, after the cathode has been inserted into the casing. The fibrous woven or nonwoven sheet may comprise a combination of fibrous material, typically comprising polyvinylalcohol fibers and cellulosic fibers. Conventional separators may also be made of multiple layers of such fibrous nonwoven material laminated to each other.
Conventional separator sheets for alkaline cells may comprise a dual layer of an ion permeable film membrane laminated to a fibrous nonwoven material. The film membrane may be of a cellulosic material typically cellophane. The cellulosic film membrane may be laminated to a fibrous nonwoven material comprising polyvinylalcohol fibers and cellulosic fibers. The cellulosic film membrane prevents zinc dendrites from penetrating into the cathode where they may cause shorting of the cell. Zinc dendrites can form in the anode during prolonged discharge. The cellulosic film membrane, however, can noticeably increase the internal resistance of the cell, particularly, in high power application. The fibrous nonwoven material provides support for the cellulosic membrane and also functions to absorb electrolyte which can be reabsorbed by the electrodes. The polyvinylalcohol fiber component in the nonwoven material withstands attack by alkaline electrolyte and lends structural support to the nonwoven layer. The cellulosic fibers gives the nonwoven much of its absorbency characteristic.
Such conventional separators which are prefabricated outside of the cell, whether of single or multiple layer, have a significant thickness, typically between about 5 mil and 10 mil (0.127 mm and 0.254 mm), more typically about 8 mil (0.203 mm). As such, they may consume a significant percentage of the internal volume of a small size cell. For example, when inserted into AA alkaline cells, such conventional separators may typically consume between about 3 and 20 percent of the useable internal cell volume by a combination of separator thickness and non-uniform conformation. Such conventional separators can only be made to conform to and cover either flat surfaces or surfaces that are only gradually and uniformly contoured, that is, surfaces which do not have steep angle of curvature or surfaces which do not have reverse curvature.
It is desirable that the separator be applied to cover cathode surfaces of essentially any shape and surface contour.
It is desirable to protect the top surface of the cathode in a manner which reduces the chance of shorting in the event that the separator develops surface irregularites along the cathode surface.
It is desirable that the separator, when applied to alkaline cells, be of minimal thickness yet durable, ion permeable, and resistant to attack by alkaline electrolyte.
It is desirable that the separator""s ionic resistance be low enough that it does not significantly impede the cell""s performance especially at high rates of discharge.
It is desirable that the separator resist passage of zinc dendrites from anode to cathode.
An aspect of the invention is directed to a method of forming a separator in an electrochemical cell having casing, anode, cathode and electrolyte, comprising the steps of coating one of the anode or cathode surfaces with a material to form a film on said surface and treating said film to form a separator contacting said electrode surface. The method of the invention is particularly suitable for forming a separator on a surface of the cathode in the cell casing of an alkaline cell. The cathode surface on which the separator is formed may be of any shape or contour. The alkaline cell may have single or multiple anode cavities. The separator may be in the form of an ion permeable film contacting and conforming to the surface of said electrode.
The method of the invention has particular application generally to forming separator film insitu on the surface of an electrode in the cell casing of bobbin type electrochemical cells, particularly bobbin type alkaline cells. Such alkaline cells, for example, may conventionally have an anode comprising zinc and cathode comprising manganese dioxide and alkaline electrolyte, or they may be in the form of conventional zinc-air cells having alkaline electrolyte, and an anode comprising zinc which is depolarized with air. Such bobbin cells are characterized by having at least one of the electrodes in the form of a discrete lump of solid or semisolid mass which is separately inserted into the cell casing. Conventional bobbin cells typically have a cylindrical casing with an open end and a closed end. After the cell contents are inserted into the casing, an end cap assembly comprising an insulating sealing disk (plug) with current collector therethrough and an end cap is inserted into the open end and the casing. The casing is crimped over the peripheral edge of the insulating sealing disk to seal the cell. The bobbin cell to which the method of the invention is applicable is distinguishable from cells wherein the electrodes are in wrapped jelly roll configuration. In the latter cells the electrodes are coated as thin layers onto conductive substrates to form a laminate with separator therebetween. The laminate is rolled into a jelly roll configuration and the rolled laminate inserted into the cell casing. The process of the invention, therefore, may be applied to forming electrode/separator laminates useful in lithium/manganese dioxide primary cells or in forming electrode/separator laminates in secondary (rechargeable cells).
An aspect of the invention is directed to providing electrical insulating protection to the top surface of the cathode to prevent shorting in the event that the insitu separator develops cracks, or surface irregularities in this region of the cell. The term cathode top surface as used herein shall mean the surface of the cathode facing the open end of the cell. The cathodes for alkaline bobbin cells are typically in the form of elongated members, for example disk or cylindrical shape having an annular region comprising cathode material. The outside surface of the cathode typically is in contact with and runs along most of the cell""s casing inside surface. The cathode inside surface typically defines a cavity which runs along the cathode length and forms the anode cavity with electrolyte permeable separator therebetween. Alkaline cells typically have one anode cavity but they can also have a plurality of separate anode cavities, each defined by separate hollow regions, typically within the cathode. The anode cavity is defined by the cathode inside surface which can be of smooth circular, semicircular shape, or elliptical shape. The cathode inside surface can also have a convoluted shape such as that defined by a continuous closed surface having alternating convex and concave portions, or irregular curvatures.
The cathode edge protector member of the invention, desirably is in the form of a disk inserted over the top surface of the cathode after the cathode has been inserted into the casing and the cathode inside surface has been coated with separator material. The top surface of the cathode is the cathode surface in closest proximity to the open end of the casing and tyically faces the casing open end. The protector disk is of material which is electrically insulating and is durable and does not corrode or deform when exposed to alkaline electrolyte. The protector disk is characterized by having a solid annular surface and at least one protruding lip extending therefrom which defines at least one hollow core running through the protector disk. The protruding lip defining the hollow core can be in the form a continuous, preferably circumferential, surface protruding from the protector disk solid annular surface. The hollow core through the protector disk, defined by said protruding lip, preferably conforms to the size and shape of the anode cavity. If the cell has a plurality of anode cavities, the protector disk can have a plurality of hollow cores running through the protector disk with each hollow core defined by a separate protruding lip. Each hollow core within the protector disk preferably conforms to the shape and size of each of the cell""s anode cavities, respectively, when viewed in cross section through a plane perpendicular to the cell"" longitudinal axis.
The protector disk annular solid surface in plan view desirably has an overall shape conforming to the shape of the cathode top surface. Preferably, the size and shape of the protector disk annular solid surface is about the same as the size and shape of the cathode top surface. The protruding lip defining the hollow regions through the disk desirably has a length between about 0.5 and 2 mm, but can be longer. After the cathode is inserted in the cell casing and separator material is applied to the exposed cathode inside surface, the protector disk is inserted preferably so that its annular surface comes to lie flush against the cathode top surface. The protruding lip penetrates into the anode cavity and comes to rest against the portion of the cathode inside surface adjacent the cathode top surface. Thus, the protector disk of the invention when inserted into the cell protects the cathode top surface and cathode corner edge at the juncture of the cathode top surface and cathode inside surface.
In another aspect the protector disk can have a raised circumferential outer edge which interlocks with a skirt protruding from the insulating sealing disk. The protector disk can also have a downwardly extending lip which covers the top inside surface of the cathode. The interlocking protector disk provides an impenetrable barrier between the anode material and the cell casing.
In another aspect the cell can be provided with an insulating sealing disk (plug) with a circumferential skirt protruding therefrom. The insulating sealing disk typically also has a rupturable membrane therein. A portion of said skirt extends over and contacts a portion of the cathode inside surface. A second portion of the skirt can contact and cover a portion of the cathode top surface. The circumferential skirt thereby covers the top inside corner edge of the cathode and provides an impenetrable barrier between the anode material and the cell casing. The insulating sealing disk in this case also functions as a cathode edge protector thereby assuring that anode material cannot penetrate passed the sealing disk and onto the cathode top surface.
The cathode protector disk of the invention is especially applicable whenever alkaline cell cathodes are coated with a polymeric film while the cathode lies insitu in the cell to form a separator film adhering to the cathode as described herein. However, the protector disk of the invention can also be used when the separators are formed exsitu, that is, outside of the cell and separately inserted in an already preformed shape into the cell.
An aspect of the invention is directed to applying a first material, preferably as a liquid or semisolid gel, on a surface of a cell electrode to form a film on said surface while the electrode is in the cell casing and then allowing the film to dry or cure insitu into a separator or treating the film with a second material, heat, light, radiation or other energy to form a separator in contact with said electrode surface. The first material may exist as a liquid or as a semisolid as defined in the Detailed Description section. If the cathode protector disk of the invention is employed, it can be inserted into the cell after the separator film is formed on the electrode.
The film on the electrode surface, which may be liquid or semisolid, may comprise polymers which may be coagulated when the second material is applied to form a solid ion permeable separator in contact with the electrode surface.
Alternatively, the film on the electrode may be dried by treating it with heated or cooled convective air, ambient air, or heated or cooled inert gas (e.g. helium or argon) or other gaseous medium or by exposing the film to a vacuum or to an infrared or other radiation source to form an ion permeable separator on the electrode. In this instance the film is composed of a polymer or compound dissolved or dispersed in a volatile aqueous or non-aqueous fluid. Alternatively, the film coated onto the electrode surface may comprise monomers or oligomers which may be polymerized to form a separator in contact with said surface. (The monomers or oligomers may be crosslinked which term as used herein is intended to be included as a form of polymerization.) The monomers or oligomers in the film coated on the electrode surface may be polymerized by applying conventional polymerization techniques such as by applying heat, light (ultra violet or visible light) ionizing radiation (e.g, gamma radiation) to the film.
Alternatively, the monomers in the film may be polymerized by contact with liquid containing free radical initiator or anionic or cationic initiator. The resulting polymers may be formed from the same or dissimilar monomers contained within the film.
Alternatively, the liquid or semisolid film on the electrode surface may comprise a fluid with a melting point higher than 60xc2x0 C. which freezes into a solid separator at room temperature. In such case the film may be applied to the electrode surface at a temperature which is above the melting point of said fluid and then the film is left to stand at room temperature whereupon it freezes into a solid separator.
A specific aspect of the invention is directed to a method of forming an ion permeable separator in an alkaline cell, comprising the steps of coating a surface of the cathode (or anode) insitu (while in the cell casing) with a first material which may be a liquid mixture comprising a polymeric material (which may be pre-catalyzed) to form a liquid or semisolid film (e.g. gel) on a surface of said cathode (or anode), and contacting the liquid or semisolid film with a second material (which may be a liquid or gaseous medium or air) or exposing the film to an energy source causing the film to dry, coagulate or polymerize into a solid or semisolid, ion permeable film separator contacting said surface. Thus, the separator made by the process of the invention may be a solid or semisolid during operation of the cell as it is exposed to normal ambient conditions, e.g. between about xe2x88x9220xc2x0 F. and 120xc2x0 F. (xe2x88x9228.8xc2x0 C. and 48.8xc2x0 C.).
Particulate filler, preferably, zinc oxide particles may be added to the liquid or semisolid mixture forming said liquid or semisolid film on the electrode surface to increase the porosity and absorbency of the final separator film. The alkaline cell preferably has a casing, an anode comprising zinc and a cathode comprising manganese dioxide, and an electrolyte comprising potassium hydroxide. The formation of an ion permeable separator in this manner allows for the formation of very thin separators, for example, having a uniform thickness of between about 1 mil and 4 mil (0.0254 mm and 0.102 mm), preferably between about 1 and 3 mils (0.0254 mm and 0.0762 mm). Such thin separators make available significantly more internal volume of the cell in smaller size cells, for example, AA size or smaller cells. The saved cell volume can be used to supply the cell with additional amount of active material or electrolyte to increase cell energy or power capacity. Such separators made by the insitu process of the invention will readily conform to, cover and adhere to a cell electrode surface, particularly an alkaline cell cathode surface, of essentially any shape and contour including (but not by way of any limitation) any flat, circular, curved, dimpled, scraped surfaces as well as any surfaces having reverse curvature. Internal cell design should not be limited to electrode shapes which can be covered by premanufactured and precut separators. The separators made by the insitu process of the invention can be applied to any shaped electrode, thereby not restricting the cell designer to any particular shaped electrode.